The present invention is concerned with a process for selectively separating and recovering silver and/or gold from predominantly chloride solutions which may contain as main constituents further heavy metals such as, for example, zinc, nickel, cobalt, iron, magnesium, and/or aluminum.
Such solutions are obtained, first, in the primary wet chemical treatment of sulphide non-ferrous-metal-containing concentrations, stones, and similar sulphide starting materials. For the purpose of recovering the non-ferrous metals, these materials are subjected to oxidizing leaching in predominantly chloride solutions. O.sub.2, Cl.sub.2, Fe.sup.3+, Cu.sup.2+, etc., may be used as oxidizing agents. Ferric chloride leaching of copper concentrates in an alkali or alkaline earth metal chloride solution is an example of the wet chemical method of treating sulphide concentrates. This method cannot prevent considerable amounts of silver, e.g. 60% of the silver in the starting material, being dissolved in addition to the copper and the iron (see U.S. Bur. Mines, Rept. Invest. 7474).
Secondly, such solutions are also obtained in the wet chemical further treatment of leach residues which are formed in the primary hydrometallurgical treatment of non-ferrous metal concentrates, e.g. in the sulphatizing roasting of sulphide copper concentrates and of complex concentrates, or in the electrowinning of zinc from zinc blende concentrates. Besides the non-ferrous metal residues not extracted, these leach residues often contain other metals which cannot be extracted during the primary extraction process, e.g. lead, silver, and gold. In many cases, the further treatment of these residues for recovering these metals still contained is required or attractive because of either the high metal value content or the higher metal yield. One way of treating these leach residues in the wet chemical manner is described in DT-OS 20 60 408 where the residue is leached in a predominantly chloride solution, with an oxidizing agent, e.g. Cl.sub.2, being added if necessary.
Thirdly, such solutions are also obtained in the treatment of pyrites cinders which, for the purpose of producing iron ore, are subjected to chlorinating sulphatizing roasting with subsequenet leaching, or in the working up of non-ferrous-metal-containing leaching residues by means of chlorinating sulphatizing roasting (see DT-OS 24 28 793).
The chloride sulphate solutions, thus formed, generally contain the non-ferrous metals Cu, Zn, Co, Ni, Ag, and, as is well known, gold, too, if Cl.sub.2 is introduced during the leaching step.
Fourthly, such solutions are also obtained by the anodic oxidation of precious-metal-containing metal anodes in chloride or in chloride sulphate solutions. As is well known, during nickel refining electrolysis there are anodically dissolved in the Hybinette cell considerable amounts of silver together with nickel and other impurities.
There are known to be various methods of working up the solutions thus obtained.
There are various ways of working up the solutions obtained by the processes first mentioned above.
In a first process the precious metals, particularly the silver, from a Cu-containing chloride solution are cemented by means of metallic iron together with the copper. The cement copper, thus obtained, must be subjected to an electrorefining step for the purpose of recovering the silver and electrolytic copper (see U.S. Bur. Mines, Rept. Invest. 7474).
In another process, a cupriferous chloride solution is first subjected after leaching to partial preliminary reduction prior to being partly decopperized with diaphragm cells during electrowinning. Thus there is obtained a silver-containing copper powder which, as in the process mentioned previously, must be subjected to another refining step (see U.S. Pat. No. 3,785,944; U.S. Bur. Mines, Rept. Invest. 8007).
In both processes it is impossible for the silver to be selectively recovered or, because of the presence of the silver, for copper of electrolytic copper quality to be recovered by means of simple pyrometallurgical refining. Hence, the copper must be subjected to additional electrorefining at considerable cost.
In another process, the cupriferous chloride solution is treated, after leaching, with cement copper in order to reduce the cupric ions contained therein. Thus, the elements selenium, tellurium, bismuth, lead, and silver are at least partly separated from the cuprous chloride solution (see U.S. Pat. No. 3,798,026). As the silver in this concentrated chloride solution is present as a silver chloride complex, the cementation of the silver is incomplete even if there is a high excess of metallic copper.
For the purpose of working up the solutions which were the second group to be mentioned above, various processes have been developed for recovering the non-ferrous metals contained therein, particularly lead, silver, and, possibly, gold.
According to one of these processes, the lead and, simultaneously, the non-ferrous metals are removed from the solution by means of precipitation as Pb(OH)Cl, with a neutralizing agent, e.g. lime water, being added (see DT-OS 25 00 453), or by means of precipitation as PbCO.sub.3, with soda being added. A reducer being added, the precipitate is remelted to base bullion. This process does not represent any selective recovery of silver and gold as against lead, so that the base bullion has to be subjected to a further refining step.
According to another process, the silver, together with the copper, is selectively precipitated as against the lead in the form of sulphide, with hydrogen sulphide being added; it is then separated from the solution (see Belgian Pat. No. 839,867). This process is characterized by a selective separation of the silver as against the lead, but not as against the copper.
An alternative to the above-mentioned processes is offered by cementation of the non-ferrous metals with the aid of either iron dust (see Erzmetall 29, 1976, 2, pp. 73-75) or lead metal (see DT-OS 20 60 408) or zinc dust. However, this cementation is not selective as against copper. If it is carried out with lead metal or in the presence of lead, the cementate contains no inconsiderable amount of lead.
For the purpose of working up the third group of solutions mentioned above, cementation with metallic iron is generally applied, with the precious metals and the cement copper being recovered simultaneously. A well known alternative to this cementation is offered by the reduction of the solutions with cement copper, with the bulk of the copper being precipitated as cuprous chloride. The residual copper is cemented with iron. The precious metals are thus conveyed into the cuprous chloride (see Lehrbuch der Metallhuttenkunde, Vol. 1, by V. Tafel, 1951, page 529).
Hence, the above-mentioned processes do not effect any selective recovery of the precious metals or any recovery of an intermediate copper product free from precious metals.
According to a process for recovering an intermediate copper product freed from the silver, the solution from the working up of pyrites cinders according to the process of chlorinating sulphatizing roasting is treated with a water-soluble iodide solution, e.g. KI or ZnI.sub.2 (see Claudet, British patent 1870, No. 282). The precipitate with the silver as silver iodide has the disadvantage of also containing considerable amounts of copper (see British patent 1870, No. 282) and considerable amounts of lead (see H. Wedding in Eisenhuttenkunde Vol. 2, 1902, pages 513-514). Hence, there is no selectivity of this silver recovery as against the copper and the lead. Besides, there is no information on the behaviour of the gold in this process because, on the one hand, the gold should be present as colloidal gold on account of being easily reducible and, hence, hardly capable of being effectively subjected to any solid-liquid separation, while on the other hand, gold, in the presence of an excess of iodide ions, has a tendency towards being dissolved again through the formation of complexes. Besides, as there is to be no previous reduction step, the iodide is additionally used as a reducer, which is a burden to the economy of the process seeing that there is no recovering of the iodide used for reduction. Thus, the process described has three quite considerable disadvantages.
For the purpose of working up the fourth group of solutions mentioned above, there is a well-known process cementing the silver with copper powder from the nickel chloride and sulphate solution. The disadvantage is that a considerable excess of copper has to be used for a quantitative removal of the silver, there being obtained a cementate predominantly containing copper but comparatively little silver.
Moreover, for the purpose of selectively recovering precious metals from predominantly chloride solutions, there have been described processes which are based on the principle of ion exchange.
For the purpose of extracting the gold, DT-AS 15 33 131 and DT-AS 19 64 922 recommend the use of an anion exchange resin with different chelate-forming groups. These processes, it is true, do extract the gold from the solution selectively, but they do not bind the silver. Moreover, on account of the strong bond, it is difficult for the gold to be eluted from the ion exchange, so that, in order to recover the gold, the ion exchanger has to be decomposed.
According to another well-known process, the precious metals gold and silver can be selectively extracted from a chloride solution by means of the solution being conveyed via an ion exchanger with SH groups. This process, however, has the following disadvantages: first, the solution must not contain any free chlorine because, on account of that, the ion exchanger would be decomposed. As, on the other hand, gold is easily reducible in the absence of chlorine, the one requirement excludes the other. The second disadvantage is the heavy complex formation of silver in chloride solution, which is the reason why the ion exchanger can only be charged with a low amount of silver. Thirdly, the conditions of eluting the silver and the gold vary very much.
The use of activated carbon for absorbing the gold from different solutions is also well known. It is impossible for the required low gold concentration in the final solution to be obtained from chloride solutions even if there is a considerable excess of most finely divided activated carbon.
All the processes described so far have one or more of the disadvantages as follows: a lack of selectivity as against the accompanying elements; incomplete recovery of the precious metals; the varying behaviour of the precious metals to be recovered towards the process applied; and the complicated and/or uneconomical further treatment of the precious metals.